The invention relates to a method of preparing viruses for use in vaccines.
Inactivation of a virus can alter viral antigens, reducing the safety or efficacy of the vaccine. Ideally, the conditions and agent(s) for viral inactivation would selectively and irreversibly affect the viral genome.
Ethyleneimine monomer (EI) or binary ethyleneimine (BEI) are reagents used to modify nucleic acids preferentially at N-7, N-3, and N-1 of purines and to a lesser extent N-3 of pyrimidines. Alkylating agents enhance the opening of an imidazole ring of N-7 alkylated purines (e.g., guanine), thereby arresting replication. EI alkylates guanosine to form N-7 (aminoethyl)guanosine which has a higher imidazole ring opening rate than does N-7 (alkylguanosine). EI also modifies non-genomic components of the viron or nonviral biomolecules.
One undesirable side-effect of this nonspecific reactivity is the disruption of viral particles which reduces immunogenicity. Chemical modification of even a single amino acid can significantly change the resistance of protein toward proteinases and may reduce stability of vaccine during storage due to proteolysis of modified viral proteins. Preferential inactivation of protective epitopes can contribute to an imbalanced immune response and to potentiation of disease during subsequent infection. Modification of capsid components can inhibit the intracellular processing of viral proteins which are necessary for presentation of epitopes to T-cells. Third, modification of amino acid residues of viral proteins reduces viral antigenicity. Finally, chemical modification of proteins present in the initial virus-containing matter may alter their antigenic specificity. This is the primary cause of allergic reactions in humans after booster doses of inactivated rabies vaccine. Despite their inherently low selectivity, ethyleneimine monomer (EI or BEI) have been used as agents for production of the killed antiviral vaccines.